This invention relates to a novel method and apparatus for offshore drilling operations. More specifically, this invention relates to a method and apparatus for employing a concentric, high-pressure, marine riser in deep water offshore drilling where well depths have previously been restricted either because of a limitation of mud weights or because the hydrostatic head above the mud line does not allow drilling with a low margin between formation fracture pressure and pore pressure. Still further, this invention relates to gas handling in a long riser and safe well shut in, in the event of an unexpected loss of station of a dynamically positioned drilling unit.
In the past, substantial oil and gas reserves have been located beneath the Gulf of Mexico, the North Sea, the Beaufort Sea, the Far East regions of the world, the Middle East, West Africa, etc. In the initial stages of offshore exploration and/or development drilling, operations were conducted in relatively shallow water of a few feet to a hundred feet or so along the near shore regions and portions of the Gulf of Mexico. Over the years, the Gulf and other regions of the world have been extensively explored and known oil and gas reserves in shallow water have been identified and drilled. As the need for cost-effective energy continues to increase throughout the world, additional reserves of oil and gas have been sought in water depths of three to five thousand feet or more on the continental shelf. As an example, one actively producing field currently exists off the coast of Louisiana in two thousand eight hundred feet of water and drilling operations off New Orleans are envisioned in the near future in approximately three thousand to seven thousand five hundred feet of water. Still further, blocks have been leased in fields of ten thousand feet, and in the near future, it is anticipated that a desire will exist for drilling in twelve thousand feet of water or more.
Deep water exploration stems not only from an increasing need to locate new reserves, as a general proposition, but with the evolution of sophisticated three dimensional seismic imaging and an increased knowledge of the attributes of turbidities and deep water sands, it is now believed that substantial high production oil and gas reserves exist within the Gulf of Mexico and elsewhere in water depths of ten thousand feet or more. Although such formations offer substantial new opportunities, significant problems also exist.
Along the near shore regions and continental slope, oil reserves have been drilled and produced by utilizing fixed towers and mobile units such as jack-up platforms. Fixed towers or platforms are typically fabricated on shore and transported to a drilling site on a barge or self floating by utilizing buoyancy chambers within the tower legs. On station, the towers are erected and fixed to the seabed. A jack-up platform usually includes a barge or self-propelled deck that is used to float the rig to station. On site, legs at the corners of the barge or self-propelled deck are jacked down into the seabed until the deck is elevated a suitable working distance above a statistical storm wave height. An example of a jack-up platform is disclosed in Richardson U.S. Pat. No. 3,412,981. A jack-up barge is depicted in U.S. Pat. No. 3,628,336 to Moore et al.
Once in position fixed towers, jack-up barges and platforms are utilized for drilling through a short riser in a manner not dramatically unlike land based operations. It will readily be appreciated that although fixed platforms and jack-up rigs are suitable in water depths of a few hundred feet or so, they are not at all useful for deep water applications.
In deeper water, a jack-up tower has been envisioned wherein a deck is used for floatation and then one or more legs are jacked down to the seabed. The foundation of these jack-up platforms can be characterized into two categories: (1) pile supported designs and (2) gravity base structures. An example of a gravity base, jack-up tower is shown in United States Herrmann et al. U.S. Pat. No. 4,265,568. Again, although a single leg jack-up has advantages in water depths of a few hundred feet it is still not a design suitable for deep water sites.
For deep water drilling, semi-submersible platforms have been designed, such as disclosed in Ray et al. U.S. Pat. No. 3,919,957. In addition, tension leg platforms have been used such as disclosed in Steddum U.S. Pat. No. 3,982,492. A tension leg platform includes a platform and a plurality of relatively large legs extending downwardly into the sea. Anchors are fixed to the seabed beneath each leg and a plurality of permanent mooring lines extend between the anchors and each leg. These mooring lines are tensioned to pull partially the legs against their buoyancy, into the sea to provide stability for the platform. An example of a tension leg platform is depicted in Ray et al. U.S. Pat. No. 4,281,613.
In even deeper water sites, turret moored drillships and dynamically positioned drillships have been used. Turret moored drillships are featured in Richardson et al. U.S. Pat. Nos. 3,191,201 and 3,279,404.
A dynamically positioned drillship is similar to a turret moored vessel wherein drilling operations are conducted through a large central opening or moon pool fashioned vertically through the vessel amid ships. Bow and stern thruster sets are utilized in cooperation with multiple sensors and computer controls to maintain the vessel dynamically at a desired latitude and longitude station. A dynamically positioned drillship and riser angle positioning system is disclosed in Dean U.S. Pat. No. 4,317,174.
Each of the above referenced patented inventions is of common assignment with the subject application.
Notwithstanding extensive success in shallow to medium depth drilling, there is a renewed belief that significant energy reserves exist beneath water having depths of three thousand to twelve thousand feet or more. The challenges of drilling exploratory wells to tap such reserves, however, and follow on developmental drilling over a plurality of wells, are formidable. In this, it is believed that methods and apparatus existing in the past will not be adequate to economically address the new deep water frontier.
The present invention was conceived to facilitate offshore drilling in deep water. For purposes of this application, the term deep water is used to designate water having a depth of greater than two thousand, five hundred feet. The subject invention is also intended for use in ultra-deep water, that is, water having a depth greater than five thousand feet. This invention, however, should not be understood to exclude other depths of water. Specifically, the present invention can be successfully utilized in depths of water as shallow as two hundred feet. Throughout this description, the term deep water will be used to refer generally to deep water and ultra-deep water. Accordingly, deep water is any water having a depth greater than two thousand five hundred feet.
As drilling depths double and triple, drilling efficiency must be increased and/or new techniques envisioned in order to offset the high day rates that will be necessary to operate equipment capable of addressing deep water applications. Drillers have found areas in deep water, wherein the soil fracture gradient is often close to the pore pressure within a few thousand feet of the sea floor. These wells can be not be drilled with conventional equipment. Underbalanced drilling which has been successfully used onshore may be the only method to drill such formations. However, underbalanced drilling from a deep water floating drillship has not been possible because of limitations in a subsea rotating blowout preventor.
In addition to low margins between fracture and pore pressures and a need in some instances for underbalanced drilling, long riser strings in deep water present gas handling problems. Still further, with a dynamically positioned drillship, it is always a possibility that through one or more system failures position stability may be lost. For safety considerations, it is necessary to provide a rapid, fail-safe riser system to accommodate vessel drift within fifteen to thirty seconds of a failure event.
The difficulties suggested in the preceding are not intended to be exhaustive, but rather are among many which may tend to reduce the effectiveness and capacity to drill offshore from a drillship in deep water. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that methods and systems for drilling in deep water from a dynamically positioned drillship will admit to worthwhile improvement.